There is a known NC automatic lathe that includes feed screws along two mutually orthogonal axis directions on a plane orthogonal to a z axis, which is along a center line of a spindle, and that moves a tool post on an orthogonal X-Y coordinate system inclined against an orthogonal A-B coordinate system consisting of the axial directions of the feed screws to machine a workpiece (see Japanese laid-open patent application No. H7-308803, for example). In this case, a plurality of tools arranged along a Y axis direction of the X-Y coordinate system moves in an X axis direction of the X-Y coordinate system to cut into the workpiece and perform machining.
Also, Japanese laid-open patent application No. 2001-300802 discloses a method for adjusting a blade edge position of respective cutting tools provided on a comb-toothed tool post of a machine tool, such as an NC lathe, against a material to be machined on the basis of machining data. According to this method, when the NC lathe machines a bar, the comb-toothed tool post, on which cutting tools used for machining operations have been mounted, is moved so that a blade edge of a particular cutting tool whose blade edge position is to be adjusted comes into contact with a lateral desired point of the bar held at an actual machining position. The lateral desired point is circumferentially approximately 90 degrees away from a vertex of the bar, the vertex being a point with which the blade edges of respective cutting tools come into contact in machining operation. On the basis of a Y coordinate value of the lateral desired position and a premeasured radius of the bar, a Y coordinate value (absolute value) of the center, or rotational axis, of the bar is determined. Then, the blade edge of the particular cutting tool is moved to the Y coordinate value of the rotational axis of the bar so that it can come into contact with the vertex of the bar to perform a cutting operation. This enables highly accurate machining.
As shown in FIG. 10A, the above-described blade edge position adjustment is difficult to execute against a bar having a radius larger than the radius Ra of a bar W1 that abuts an adjacent cutting tool, because the adjacent cutting tool interferes with the execution of the blade edge position adjustment. Thus, as shown in FIG. 10B, in a machine tool disclosed in Japanese laid-open patent application No. 2001-300802, blade edges of respective tools 220 are staggered, in a stepwise fashion, in a cut-in direction of the tools 220, which is orthogonal to the arrangement direction of the tools. Thus, the blade edge position adjustment can be executed against a bar W2 having a radius Rb that is larger than the distance between two adjacent tools.